The optical device is employed in various devices, including: information display devices with dot matrix display such as monitors of portable phones, in-vehicle systems, or household electric appliances, display devices for personal computers, and TV receivers; fixed display devices such as clocks or advertising panels; illumination devices such as a power source of a scanner or a printer, lighting, or LCD backlight; and optical communication devices that use photoelectric conversion functions. An optical device generally consists of multiple pixels such that by switching on or off each of the pixels, predetermined information is displayed. Some known optical devices employ self light emitting elements for the pixels. A self light emitting element has the advantages of low power consumption and no need for a backlight and has found various applications ranging from small-scale optical devices to larger ones such as large screens, including optical panels with multiple self light emitting elements arranged in a dot matrix, displays with icons (fixed display parts), and flat or spherical lighting equipment.
Self light emitting elements include inorganic EL elements, organic EL elements, FED elements, and light emitting diodes; an organic electroluminescence (EL) element is the typical example. An organic EL element is also referred to as an organic electroluminescence (OEL) device, an organic light emitting diode (OLED) device, a self light emitting element, or an electroluminescence light source. An organic EL element generally has a structure in which an organic layer (corresponding to a light emitting layer) is interposed between an anode (hole injecting electrode) and a cathode (corresponding to an electron injecting layer). The organic layer usually has a multilayer structure, each layer having its own function, and for example, a hole injecting layer, a hole transporting layer, an organic light emitting layer, an electron transporting layer, an electron injecting layer, and others are layered upon one another in consecutive order. Each of these layers may be a single layer made of a single organic material, a mixed layer of a mixture of several materials, or a layer containing organic or inorganic materials having respective functions (charge transporting function, light emitting function, charge blocking function, optical function and the like) dispersed in a polymer binder. Some other known organic EL elements include a buffer function for each layer to prevent the organic layer from being damaged during formation of an upper electrode by a sputtering method, or a planarizing function to reduce surface roughness resulting from film forming processes.
In the organic EL element having the above structure, when a voltage is applied across the electrodes, holes injected from the anode and transported into the organic layer and electrons injected from the cathode and transported into the organic layer recombine in the organic layer. The recombination causes a transition of electrons of the organic molecules in the organic layer from the ground state to an excited state, and the transition of the electrons from the excited state back to the ground state results in emission of light.
The color of light emitted from an organic EL element is substantially defined by the material of the organic layer, which is the light emitting layer. Various techniques are known for emitting light of a predetermined color, including: forming the organic layer from a luminescent material that emits light of a predetermined color; a filtering method wherein light from the organic layer is emitted through a color filter; a color conversion method wherein a fluorescent layer absorbs light from the organic layer and emits light of a predetermined color; using various luminescent materials mixed in one organic layer; and an additive color mixing method that uses mixing of colors to achieve light of a predetermined color (see for example Patent Document 1). The additive color mixing method has the advantage that a predetermined color other than the intrinsic color of the organic layer can be achieved with a simple structure without the need of providing a filter or a fluorescent layer.
A conventional light emitting panel device that emits light of a predetermined color (for example white) using this additive color mixing method controls driving of pixels of various types of colors (for example red pixels, blue pixels etc.) to emit light of a predetermined mixed color (for example white light).
Patent Document 1: Japanese Published Unexamined Patent Application No. 2003-123971